Angiogenesis is the formation of new blood vessels from preexisting endothelial vasculature. Folkman, et al., J. Exp. Med. 133:275-288, (1971). Most tumors require angiogenesis to sustain growth beyond a critical volume of 1-2 mm, when the supply of nutrients and metabolites becomes insufficient due to the limits of diffusional exchange. Folkman, J. Nat. Cancer Inst. 82:4-6 (1990). Tumors deprived of angiogenesis remain dormant indefinitely, only to rapidly grow when a blood supply is acquired. Brem et al., Cancer Res. 36:2807-2812 (1976). The degree of angiogenesis often increases with tumor progression. Dome et al., J. Pathol. 197:355-362 (2002). Further, invasion and metastatic spread of tumors are also thought to be angiogenesis-dependant events. Folkman, Ann Surg. 175:409-416 (1972). The newly formed blood vessels provide a route for cancer cells to enter the circulatory system and spread to distant parts of the body. Fidler and Ellis, Cell 79:185-188 (1994).
Because angiogenesis is an integral process in the growth and spread of tumors, it is an important focus of cancer therapy. Anti-angiogenesis therapy is effective not only for solid tumors, but also hematopoietic tumors, leukemia and myeloma, Bellamy et al., Cancer Res. 59:728-733 (1999); Rajkumar et al., Leukemia. 13:469-472 (1999). Endothelial cells are thought to be better targets for therapy than tumor cells because they have a longer generation time and more genetic stability that tumor cells. Endothelial cells are therefore less likely to “escape” therapy by developing drug resistance to the therapy administered. Boehn-Vaiswanathan, Curr. Opin. Oncol. 12:89-94 (2000).
Other conditions suffered by mammals are also related to inappropriate angiogenesis. Wet macular degeneration occurs when blood capillaries inappropriately grow into the retina. Inappropriate angiogenesis has also been implicated as a fundamental characteristic of diabetic retinopathy, psoriasis and rheumatoid arthritis, among other diseases. Bussolino et al., Trends Biochem. Sci. 22:251-256 (1997); Folkman, Nat. Med. 1: 27-31 (1995).
What is needed are additional therapies for inappropriate angiogenesis, particularly that which occurs during tumor formation. Such therapies may be useful in many conditions that exhibit inappropriate or unwanted formation of new blood vessels.